IN 1930 a radium pack was designed and constructed as described in two previous papers (1, 2) to contain four grams of radium. The radium was distributed in 100-milligram tubes over an area 8 × 8 cm., each radium tube radiating the entire field of 10 × 10 cm., at either 6, 10, or 15 cm. radium-skin distance. The large radiating area and field size results in extreme spreading of the beam after it leaves the pack, especially at the shorter distances. This characteristic is a decided advantage when treating large lesions located at or near the surface. When treating deep-seated lesions, however, such as those of the mid-pelvic region, this wide spreading of the beam prevents a close spacing of adjacent fields, particularly when multiple ports of entry are used to produce a cross-firing effect at depth. It was found that with this pack at a radium-skin distance of 10 centimeters, two anterior and two posterior ports could be used to advantage, provided a distance of 5 centimeters was maintained between the ports on each skin surface, and that the angle did not exceed 15°. This arrangement produced a depth intensity of 63 per cent at the midpoint of a pelvis 20 centimeters antero-posterior dimension. Because of the field size and the space required between adjacent fields, it was impossible to apply more than two fields with this pack to either the anterior or the posterior surfaces of such a pelvis. Two lateral fields could, of course, be used in addition to the two anterior and posterior fields, but the distance from these lateral fields to the interior was too great to cause a sufficient increase to warrant their use; moreover, the skin areas on those surfaces adjacent to the treatment area would receive considerable radiation from the lateral port, due again to the extreme flare of the beam. Actual estimation by means of isodose charts, of the intensity at depth with two anterior, two posterior, and two lateral ports, showed that the intensity at the 10 cm. depth had increased to only 66 per cent, with all skin areas receiving 100 per cent at a radium-skin distance of 10 centimeters. Greater intensities than these were desired, in order to effectively radiate the deep-seated lesions. Of course, a greater intensity could be obtained for mid-pelvic regions, by using the pack at a distance of 15 centimeters, but the treatment time would be lengthened to such an extent, especially with multiple ports, that this method was found to be impractical with the amount of radium available. Furthermore, at this distance, more radiation penetrates to the opposite skin surface, which necessarily reduces the dose which can be applied to that skin surface and, therefore, reduces the dose in the mid-pelvic region. In order to produce a greater intensity at depth by the utilization of more ports of entry, it was decided to remove the radium from the 4-gram pack, and redistribute it in a pack of radically different design and purpose.